The sound of a guitar playing through a classic 1960's Fender or Marshall vacuum tube amplifier and speaker, such as a Celestion 12 inch model G12, becomes identified with Rock and Roll music, and in essence defined a new form of musical instrument. Rather than simply a means for providing increased loudness for entertaining large audiences, those "classic" rock and roll guitar vacuum tube amplifiers and associated output transducers provided a reedlike sound when driven into overload which could be finely controlled and shaded into a variety of musical textures by the skilled guitarist. Various attempts have been made over the past twenty years to simulate that highly desirable reedlike sound by means of transistorized amplifiers driving speakers or driving resistors, or by means of vacuum tube amplifiers driving resistors.
Laub, in U.S. Pat. No. 3,835,409 teaches a solid state amplifier which includes means for generating large amounts of crossover distortion in proportion to the amplitude of the signal. Moog, in U.S. Pat. No. 4,180,707, teaches a variety of solid state circuits which can create even or odd order distortion to simulate some of the effects of overdriven vacuum tubes. Smith, in U.S. Pat. No. 4,211,893 teaches an electronic amplifying apparatus which is intended to simulate the distortions of vacuum tubes by the use of either FETs or bipolar transistors. Sondermeyer, in U.S. Pat. No. 4,405,832 teaches a distortion circuit consisting primarily of a diode clipper and a potentiometer incorporated in an operational amplifier circuit. An article by T. E. Rutt entitled "Vacuum Tube Triode Nonlinearity as part of the Electric Guitar Sound." which was published as Audio Engineering Society Preprint 2141, October 1984 discloses that the distinctive distortion of a tube amplifier is caused by grid current effects which have never been successfully duplicated by semiconductor circuits. Scholtz has sold a commercial product under the brand name Power Soak which resistively loaded a guitar amplifier and provided for an adjustable portion of the electrical output signal which can then be applied to headphones, a speaker, or a recording console; this allows an amplifier to be driven into overload safely while not having to produce loud sounds in air.
All of these prior art devices and inventions have shortcomings which prevent them from providing all of the touch and sound of the classic tube-type guitar amplifier/speaker/cabinet combination as an electrical signal which can be fed directly into a conventional recording console and/or transduced into musical sounds in air by conventional linear means.
Another prior art method which can under certain circumstances provide all of the touch and sound of the classic tube-type guitar amplifier/speaker/cabinet combination as an electrical signal was to place a microphone in front of a conventional loudspeaker contained in a classic small tube type Fender, Vox, or Marshall amp, and then use the microphone to transduce the musical sounds from the loudspeaker into electrical signals which can then be further amplified, recorded, and again transduced into sounds in air either at a later time or at a higher amplitude. That technique required that loud performance level sounds to be generated in air, was susceptible to undesirable feedback in a real-time reinforcement situation, and was subject to undesirable variations in sound quality as a function of microphone type, placement, and the room in which the instrument is placed. It is also susceptible to contamination by outside sounds and noises.